Research suggests men who sleep with women suffer impaired mental performance and increased stress.

The Austrian researchers analysed the sleep patterns of young couples, comparing their sleep and after sleep performance when sleeping alone versus sleeping together with their partner.

Comparing 10 nights in each configuration for each of the couples, they found men suffered increased stress levels and exhibited poorer cognitive test performance when they had not slept alone, an effect observed irrespective of whether the couple had sex together.

Women are also said to have had their sleep and performance impaired by sleeping with their partner, though to a much lesser extent than with men.

If we want to be serious though, it's not really surprising that you have impaired mental performance if you spend the night sleeping next to someone, as I expect your sleep quality would be worse. It is interesting that women suffer from it less than men. Perhaps they are deeper sleepers in general?